Systems, Methods, and Apparatus for Rub Detection in a Machine

ABSTRACT

Systems, methods and apparatus for monitoring rub detection in a machine are provided. An electrical signal may be provided for transmission to and into a component of the machine. A capacitance associated with the electrical signal in the component may be monitored. Based at least in part upon a determined change in the monitored capacitance, a potential rub condition for the component of the machine.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/417,996, entitled “Systems, Methods, and Apparatus for RubDetection in a Machine,” which was filed on Apr. 3, 2009, and isincorporated herein by reference.

TECHNICAL FIELD

Embodiments of the invention relate generally to machines, and morespecifically, to rub detection in a machine.

BACKGROUND

Many machines, such as turbines, have a centrally disposed rotatingportion that rotates within a stationary portion, casing or shell. Inturbines for example, a typical problem known as “rubbing” arises when arotating portion of the turbine, such as a rotor, comes into contactwith another component of the turbine, such as the stationary caging,when the components are not in contact with each other under normalcircumstances. Rubbing often gives rise to unwanted and potentiallydangerous conditions in rotating machinery. For example, one of theharmful effects of rubbing can be witnessed in steam turbines.Typically, steam turbines include a rotor with rows of turbine bladesthat rotate between rows of stator blades. The tips of the turbineblades are adjacent to an inside surface of the casing for the turbine.During normal operation, the tips of the turbine blades do not rubagainst the casing. However, a slight deformity in the turbine casing,rotor shaft, inner casing or any other component can cause the turbineblades to rub against the stationary casing of the turbine. Rubbingbetween the turbine blades and the stationary component in a steamturbine can damage the turbine components, while increasing theclearance between the turbine blades and the stationary casing toprevent rubbing can reduce the efficiency of the flow through the steamturbine.

Rubbing may occur due to an unbalance in the rotor, thus giving rise toabnormal vibrations in the turbine. In conventional turbines, aplurality of vibration sensors may be mounted at various points on theturbine system to monitor signature vibrations indicative of rubbing.Other techniques for monitoring rubbing may include the use ofparticular sensors for detecting the occurrence of acoustic emissionswithin the metal parts of the turbine, such acoustic emissions beinggenerated as a result of certain abnormal operating conditions in theturbine. Such conventional techniques, however, have often proved to beinaccurate as it is extremely difficult to distinguish between thevibrations caused by rubbing and the vibrations caused by the turbineitself. More specifically, the vibrations detected by the sensorsusually include the vibrations resulting from the various components ofthe turbine itself.

Accordingly, there is a need for methods, systems and apparatus fordetection of rubbing in machines.

BRIEF DESCRIPTION

According to one embodiment of the invention, there is disclosed amethod for monitoring a machine. The method may include providing anelectrical signal for transmission to a component of the machine, suchas a rotating component of a turbine. A capacitance associated with theelectrical signal may be monitored, and a change in the monitoredcapacitance may be determined. A potential rub condition for thecomponent of the machine may be detected based at least in part on thedetermined change in the monitored capacitance.

According to another embodiment of the invention, there is disclosed asystem for monitoring a machine. The system may include a signalgenerator, one or more detection devices, and at least one processor.The signal generator may be configured to provide an electrical signalthat is transmitted to at least one component of the machine, such as arotating component in a turbine. The one or more detection devices maybe configured to detect a capacitance associated with the electricalsignal in the at least one component of the machine. The at least oneprocessor may be configured to compare the monitored capacitance to atleast one expected value; and detect, based at least in part on thecomparison, the occurrence of a potential rub condition for thecomponent.

According to yet another embodiment of the invention, a method formonitoring a machine is disclosed. An alternating current voltage signalmay be transmitted to at least one component of the machine, such as arotating component in a turbine. A capacitance associated with thesignal may be monitored, and the determined capacitance may be comparedto an at least one expected value. Based at least in part on thecomparison, the occurrence of a potential rub condition between the atleast one machine component and another component of the machine may bedetected.

Other embodiments, aspects, and features of the invention will becomeapparent to those skilled in the art from the following detaileddescription, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a schematic view of one example system for monitoring amachine, according to an illustrative embodiment of the invention.

FIG. 2 is a magnified cross-sectional side view of a portion of aturbine that may be monitored in accordance with various embodiments ofthe invention.

FIG. 3 is a flowchart illustrating one example method for monitoring amachine, according to an illustrative embodiment of the invention.

FIG. 4 is a diagram illustrating one example of a change in thecapacitance associated with a rotating component of a turbine during apotential rub condition and a rub condition, in accordance with anembodiment of the invention.

FIG. 5 is a diagram illustrating one example of a change in theresistance and capacitance associated with a rotating component of aturbine during a potential rub condition and a rub condition, inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION

Illustrative embodiments of the invention now will be described morefully hereinafter with reference to the accompanying drawings, in whichsome, but not all embodiments of the invention are shown. Indeed, theinvention may be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will satisfy applicablelegal requirements. Like numbers refer to like elements throughout.

Disclosed are systems, methods and apparatus for detection of rubbingand/or potential rubbing between machine components. One example of amachine that may be monitored by certain embodiments of the invention isa turbine with at least one rotating component and stationary component;however, other embodiments of the invention may be utilized to monitorother types of machines including components which may come in contactwith each other. A signal generator may generate an electrical signalthat is provided to the at least one component, such as the rotatingcomponent of the turbine. The electrical signal may be monitored on therotating component and/or one or more measurements associated with theelectrical signal may be made. One or more characteristics associatedwith the monitored electrical signal may be determined or identifiedbased at least in part on the monitored and/or measured values. Forexample, an impedance associated with the electrical signal may bemeasured and/or determined in real time or near real time. The one ormore characteristics may be compared to one or more expected valuesand/or ranges of expected values. For example, the determined impedancemay be compared to an expected value or range of values for theimpedance. Based at least in part on the comparison, a rub or apotential rub condition between the rotating component and anothercomponent of the turbine may be identified.

FIG. 1 is a schematic view of one example system 100 for monitoring aturbine 102, according to an illustrative embodiment of the invention.The example system 100 may be used for monitoring components of any typeof machine; the turbine 102 shown in FIG. 1 being one example of amachine which may be monitored. In FIG. 1, a cross-sectional view of anexample steam turbine 102 which may be used in a variety ofapplications, for example, power generation applications, isillustrated. Although a steam turbine is illustrated, embodiments of theinvention may be utilized in association with a wide variety ofdifferent turbines and/or types of turbines, including but not limitedto, steam turbines, gas turbines, etc.

The steam turbine 102 may include a rotating component, such as a rotor104, enclosed in a stationary component with multiple turbine casings,such as an outer turbine casing 106 and an inner turbine casing 108. Aplurality of rotating turbine blades 110 may be mounted on the rotor104, and may extend radially towards the inner turbine casing 108. Theserotating turbine blades may be placed between adjacent rows ofstationary stator blades 112, which extend radially inward from theinner turbine casing 108 and towards the rotor 104. During normaloperation of the steam turbine 102, steam enters the turbine 102 from aboiler through at least one inlet nozzle and expands as the steam passesthrough the rotating turbine blades 110 and the stationary stator blades112. However, lateral vibration of the rotor 104 during the operation ofthe steam turbine 102 may lead to rubbing between the tips of therotating turbine blades 110 mounted on the rotor 104, and one or moreother components of the turbine 102, such as the inner turbine casing108. Thus, at least one detection device 114 (herein after referred toas rub detection unit 114) operable to detect a rub condition or apotential rub condition between the rotor 104 and the inner turbinecasing 108 may be utilized in association with the steam turbine 102.

Further, as shown in FIG. 1, the system 100 may include a suitablesignal generator 116 in communication with the steam turbine 102. Forexample, a signal generator 116 may be mounted on the outer surface ofthe turbine section 110 of the steam turbine 102. As another example,the signal generator 116 may be situated remotely to the steam turbine102. The signal generator 116 may provide an electrical signal to one ormore rotating components of the steam turbine 102. Examples of thesignal generator 116 may include a voltage source, current source,function generator, frequency generator, waveform generator and thelike.

Additionally, as shown in FIG. 1, a rub detection unit 114 incommunication with or otherwise associated with the steam turbine 102may be provided. The rub detection unit 114 may be a processor drivendevice that facilitates the detection of rub conditions and/or potentialrub conditions. The rub detection unit 114 may facilitate monitoring ofthe electrical signal on the one or more rotating components. The rubdetection unit 114 may determine one or more characteristics associatedwith the monitored electrical signal and, based at least in part on acomparison of the one or more characteristics to one or more respectiveexpected values and/or ranges of expected values, the rub detection unit114 may detect a rub condition or a potential rub condition. In certainembodiments, the rub detection unit 114 may be in communication with oneor more suitable sensors or sensing devices that measure the electricalsignal on one or more rotating components. Examples of suitable sensorsor sensing devices include, but are not limited to, voltage sensors,current sensors, etc.

As shown in FIG. 1, the rub detection unit 114 may include one or moreinterface devices 118, such as, network ports, etc. that facilitatereceiving data in the form of electrical signals. These one or moreinterface devices may facilitate the receipt of measurements associatedwith the electrical signal on the one or more rotating components.Additionally or alternatively, the rub detection unit 114 may includeone or more suitable analog to digital (A/D) converters 120 thatfacilitate the receipt of analog measurements data associated with theelectrical signal and the conversion of the analog data into digitaldata that may then be processed at a processor 122. The A/D converters120 may also facilitate the conversion of any analog signals that arereceived through the one or more interface devices 118 as desired invarious embodiments of the invention. The rub detection unit 114 mayalso include a memory 124 in communication with the processor 122. Thememory 124 may facilitate storage of data 126, such as one or more ofthe characteristics associated with the monitored electrical signal. Thememory 124 may also include programmable logic and/or softwarecomponents that may be executed by the processor 122, such as, anoperating system 128.

FIG. 2 is a magnified cross-sectional side view of a portion of aturbine 200 that may be monitored in accordance with various embodimentsof the invention. A cross-sectional view of a steam turbine 200 is shownin FIG. 2; however, embodiments of the invention may be utilized inassociation with a wide variety of different turbines and types ofturbines. The steam turbine 200 may include an expansion stage which maybe enclosed inside a stationary assembly, such as a housing 202, whichmay surround a rotating assembly, such as a rotor 204. A plurality ofrotating turbine blades 206 may be mounted on the rotor 204, and mayextend radially towards an inner surface of the housing 202.Additionally, a row of stationary stator blades 208 fastened to thehousing 202 may be placed between and adjacent to rows of the pluralityof rotating turbine blades 206. The rotating turbine blades 206 and thestationary stator blades 208 may be preceded by a steam inlet path 210through which steam from a boiler (not shown in figure) may enter thesteam turbine 200.

The expansion stage contained inside the housing 202 may define anannular path for the hot gases coming from the boiler and enteringthrough the steam inlet path 210. The hot gases coming through the steaminlet path 210 may rotate the plurality of rotating turbine blades 206.Tips of the plurality of rotating turbine blades 206 may rotate in anapproximately circular path that is adjacent to the inner surface of thehousing 202. As the plurality of rotating turbine blades 206 rotate,contact between the tips of the plurality of rotating turbine blades 206and the inner surface of the housing 202 may occur as a result of thelateral vibration of the rotating component. This contact, or rubbing,in the turbine 200 may be caused by a wide variety of factors and/orcombination of factors, for example, rotor imbalance, misalignment ofturbine components, etc.

In various embodiments of the invention, a rub detection unit, such asrub detection unit 114 shown in FIG. 1, may be in communication with aturbine, such as turbine 102 or turbine 200. The rub detection unit 114may be operable to detect rubbing between one or more rotatingcomponents of the turbine 102, such as rotating turbine blades 110 orrotating turbine blades 206, and one or more other components of theturbine 102, such as inner housing 108 or inner housing 202. The signalgenerator 116 (shown in FIG. 1) and the nib detection unit 114 may workin conjunction with one another to detect a rub condition or a potentialrub condition in a turbine 102. The signal generator 116 may transmit orotherwise communicate an electrical signal, such as an alternatingcurrent (AC) voltage signal, to a rotating component of the steamturbine 102, for example, to a rotor of the steam turbine 102. A widevariety of signals may be communicated as desired in various embodimentsof the invention. In one example embodiment, an AC voltage signal may becommunicated with an amplitude of approximately one volt toapproximately two volts and a frequency between approximately onekilohertz and approximately 50 kilohertz.

The rub detection unit 114 may receive measurements associated with theelectrical signal provided to the rotor through the interface device(s)118 and/or through the A/D converters 120. The A/D converters 120 mayconvert incoming analog electrical signals to digital electrical signalsand the digital electrical signals may be communicated to the processor122. The processor 122 may be operable to determine one or morecharacteristics associated with the electrical signal provided to theturbine and monitored. The determination of the one or morecharacteristics may be based on the received measurements data. Anoperating system 128 included in the memory 124 may facilitate theexecution of any number of software components associated with the rubdetection unit 114. In addition to the operating system, the memory 124may also store data 126, including measurements data received,determined, and/or calculated by the processor and/or expected valuesand/or a range of expected values for the one or more characteristicsdetermined by the processor 122. In various embodiments, the values ofthe one or more characteristics determined by the processor 122 and/orstored in the data 126 may be impedance values, resistance values,and/or capacitance values. The processor 122 may utilize the operatingsystem 128 in comparing the values of the one or more characteristics tothe respective expected values, or ranges of expected values, and indoing so may utilize the data 126. For example, the one or morecharacteristics may relate to impedance, resistance, and/or capacitance.The processor may utilize the operating system 128 in comparingdetermined values of impedance, resistance and/or capacitance torespective expected values, or ranges of expected values stored in thedata 126. Based at least in part on the comparison, a rub or a potentialrub condition between the rotating component and another component ofthe turbine may be determined or identified. Further, a severityassociated with a detected rub condition may be determined based atleast in part on a length of time for which the calculated values of theone or more characteristics fall outside the expected range of values.

FIG. 3 is a flowchart illustrating one example method 300 for monitoringa machine, such as a turbine, according to an illustrative embodiment ofthe invention. The method 300 may be used for monitoring at least onecomponent in any type of machine, and may not be limited to monitoring arotating component in a steam turbine. The method 300 may begin at block305. At block 305, an electrical signal may be provided to at least onecomponent of a machine, such as a rotating component of a turbine 102shown in FIG. 1. One example of a signal that may be provided is analternating current (AC) voltage signal. A wide variety of differentsignals may be provided as desired in various embodiments of theinvention, for example, an AC voltage signal with an amplitude ofapproximately one volt to approximately two volts. A signal generator116, for example, a waveform generator, a frequency generator or afunction generator may be used for generating the electrical signalprovided to the at least one component of the machine. A wide variety ofsignals with different characteristics may be provided as desired invarious embodiments of the invention. For example, the provided signalmay be a voltage signal with a frequency of approximately one kilohertzto approximately 50 kilohertz. Following the transmission of theelectrical signal to the at least one machine component at block 305,operations may proceed to block 310.

At block 310, the electrical signal provided to the at least one machinecomponent may be monitored and/or one or more values associated with theelectrical signal may be measured. The one or more monitored values ofthe electrical signal provided to the at least one component of themachine may be measured by any suitable number of sensors or sensingdevices, for example, voltage sensors, current sensors, etc.Additionally, according to an aspect of the invention, the electricalsignal may be monitored in real time or near real time during theoperation of the machine. The electrical signal provided to thecomponent of the machine may be an AC voltage signal or an AC currentsignal for example. In one example embodiment of the invention, the oneor more measured values may be communicated to a rub detection unit,such as nib detection unit 114 shown in FIG. 1. The one or more measuredvalues may be communicated to the rub detection unit in real time ornear real time. Following the monitoring and/or measuring of theprovided electrical signal at block 310, operations may continue atblock 315.

At block 315, one or more characteristics associated with the monitoredand/or measured electrical signal may be determined based at least inpart on the measured values. In one example embodiment, the measuredvalues of the electrical signal communicated to the rub detectioncomponent 114 at block 310 may be processed by one or more suitableprocessors, such as processor 122 shown in FIG. 1. The processor 122 maybe operable to determine one or more characteristics associated with themeasured values of the electrical signal. Examples of one or morecharacteristics that may be determined are an impedance that is presenton the component of the machine or a capacitance associated with thecomponent of the machine. As one example, the impedance of the monitoredsignal may be determined by taking the ratio of the voltage of the ACsignal to the current of the AC signal. For example, a ratio may bedetermined for a measured voltage of the signal with respect to a knowncurrent for the signal. The calculated impedance may have a real partand an imaginary part. The real part of the impedance may indicate theresistance, while the imaginary part may indicate the capacitance. Theprocessor 122 may be further operable to calculate resistance valuesand/or capacitance values as desired. Alternatively, in an exampleembodiment, an oscilloscope may be coupled to the processor 122, and maybe operable to calculate impedance, resistance and/or capacitance valuesas desired. One or more of the characteristics may be determined todetect a rub condition and/or a potential rub condition of the machinecomponent. Following the determination of the one or morecharacteristics associated with the measured values of the electricsignal, at block 315, operations may continue at block 320.

At block 320, at least one of the one or more characteristics may becompared to one or more expected values and/or one or more ranges ofexpected values. In one example embodiment, a suitable memory deviceassociated with the rub detection unit 114, such as memory 124 shown inFIG. 1, may include data 126 containing expected values and/or ranges ofexpected values for the one or more characteristics. For example,expected impedance values or ranges of expected impedance values may bestored in the memory 124. As another example, expected capacitancevalues and/or ranges of expected capacitance values may be stored in thememory 124. The processor 122 may perform a comparison between thecalculated values of the one or more characteristics to the expectedvalues or ranges of expected values stored in the memory 124. Followingthe comparing of the calculated values of the one or morecharacteristics to the expected values or range of expected values atblock 320, operations may continue at block 325.

At block 325, a rub condition or a potential rub condition may beidentified or detected based at least in part on the comparison at block320. Following the comparison between the calculated values of the oneor more characteristics and the expected values or the range of expectedvalues by the processor 122, a rub condition or a potential rubcondition may be identified depending on which characteristic associatedwith the electric signal is monitored and subsequently compared.Further, in some embodiments of the invention, the severity associatedwith the detected rub condition may be determined based at least in parton a length of time for which the calculated values of characteristicassociated with the electric signal falls outside an expected range ofcharacteristic values. In one example embodiment, if the impedancevalues calculated by the processor 122 at block 315 exceed acorresponding expected value, a rub condition may be identified.Further, severity associated with the detected rub condition may bedetermined based at least in part on a length of time for which thecalculated impedance values exceed the expected values. In anotherexample embodiment, if the resistance values calculated by the processor122 or otherwise at block 315 approaches a near zero value or a valuethat is relatively low with respect to an expected value, a rubcondition may be identified. Further, the severity associated with thedetected rub condition may be determined based at least in part on alength of time for which the calculated resistance values approach nearzero values. As another example, a potential rub condition may beidentified if the capacitance values calculated by the processor 122 orotherwise at block 315, exceed an expected value or a range of expectedvalues. The determined capacitance may increase as a component of themachine approaches another component of the machine. The increase incapacitance may be detected in order to identify a potential rubcondition.

The method 300 may end following block 325.

The operations described in the method 300 of FIG. 3 do not necessarilyhave to be performed in the order set forth in FIG. 3, but instead maybe performed in any suitable order. Additionally, in certain embodimentsof the invention, more or less than all of the elements or operationsset forth in FIG. 3 may be performed.

FIG. 4 is a diagram illustrating one example of a change in thecapacitance associated with a rotating component of a turbine during apotential rub condition and during a rub condition, according to anillustrative embodiment of the invention. FIG. 4 shows a plot 400representative of an example variance in capacitance associated with arubbing condition between a rotating component, such as a rotor, andanother component of a turbine, such as a stator. In the plot 400, thehorizontal axis may represent a distance of the rotor from the statornormalized with respect to the rotor center, while the vertical axis mayrepresent the capacitance at various distances of the rotor from thestator. The capacitance values are shown as multiples of the capacitancefor when the rotor is centered. FIG. 4 shows the capacitance values tobe fairly constant over almost the entire distance from the rotor to thestator, except for a small range of distance near the stator, where theplot 400 shows a relatively steep climb or increase in capacitance. Apotential rub condition may be detected when the capacitance rises abovea threshold value. The trend in the capacitance values in plot 400indicates that the capacitance between the rotor and the stator mayincrease as the rotor approaches the stator. Specifically, during aperiod of time immediately prior to a rub condition, the capacitance mayincrease relatively quickly. A potential rub condition may be detectedbased on identifying this increase in capacitance in real time or nearreal time during the operation of the turbine. In this regard, a rubcondition may be prevented by identifying a potential rub condition andtaking an appropriate preventive action, such as, shutting down theturbine, decreasing the load in the turbine, etc. Although, thevariation in capacitance for any component of a machine may not exactlyreplicate the trend shown in FIG. 4, the change in capacitance betweenany two components of a machine in a potential rub condition may besimilar.

FIG. 5 is a diagram illustrating one example of a change in theresistance and capacitance associated with a rotating component of aturbine during a potential rub condition and during a rub condition,according to an illustrative embodiment of the invention. FIG. 5 shows aplot 500 representative of an example variance in capacitance andresistance associated with a rub condition or a potential rub conditionbetween a rotating component, such as a rotor, and another component ofa turbine, such as a stator. In the plot 500, the horizontal axis mayrepresent time, while the vertical axis may represent the normalizedvalues of capacitance and resistance at various instants of time. Atinitial time, rotor is centered with respect to stator. The plot 500shows that as we move away from the origin of the axes with time, therotor moves towards the stator. The rotor subsequently rubs the statorafter a first range in time. After a second range in time, the rotormoves away from the stator, and moves towards the centered position fora third range in time. Thus, the final time indicates the time at whichthe rotor is again centered with respect to the stator. As shown in theplot 500, the resistance and capacitance values remain almost constantall throughout until the rotor approaches the stator. The capacitancevalues show a relatively steep climb just prior to rub, while theresistance values show a relatively dramatic decrease in the region nearthe stator, and approach near zero values in the second range in timewhen the rotor rubs against the stator. Thus based on at least one ofthe capacitance or the resistance values, or a combination thereof, arub condition and/or a potential rub condition may be identified.Additionally, severity associated with a rub condition may be determinedbased at least in part of on the length of time for which the resistancevalues approach near zero values. Thus, the length of the second rangein time for which the rotor rubs the stator partly determines theseverity associated with the rubbing. Though the variation incapacitance and/or resistance between any two components of a machinemay not exactly replicate the trend shown by the rotating component ofthe turbine in FIG. 4, the change in capacitance and/or resistancebetween any two components in a potential rub condition may be similar.

Embodiments of the invention may be applicable for any device thatincludes one or more electrically conductive components that may contactone or more other components of the device. Some examples of suchdevices include but are not limited to compressors, gas turbines, steamturbines etc. The invention may be broadly applicable to any devicewhich includes a plurality of components that may come in contact witheach other. It will be apparent that any example taken provided in theforegoing specification is merely provided for explanation purposes anddoes not limit the scope of the invention by any means.

The technical effect of embodiments of the invention is to monitor amachine, such as a turbine and determine or otherwise identify rubconditions or potential rub conditions within the machine. Varioussoftware components and/or algorithms may be utilized in embodiments ofthe invention in order to identify rub conditions and/or potential rubconditions.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scopethe invention is defined in the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1. A method for monitoring a machine, the method comprising: providingan electrical signal for transmission to and into a component of themachine; monitoring a capacitance associated with the electrical signalin the component; determining a change in the monitored capacitance; anddetecting, based at least in part on the determined change, a potentialrub condition for the component of the machine.
 2. The method of claim1, wherein the machine comprises a turbine.
 3. The method of claim 1,wherein the electrical signal comprises an alternating current voltagesignal comprising a voltage of approximately one to approximately twovolts
 4. The method of claim 1, wherein detecting a potential rubcondition further comprises determining that the monitored capacitanceexceeds a threshold value.
 5. The method of claim 1, further comprising:monitoring an impedance associated with the electrical signal, anddetecting a rub condition based at least in part on determining adecrease in the monitored impedance.
 6. The method of claim 5, furthercomprising determining a severity associated with the detected rubcondition based at least in part on a length of time for which themonitored impedance falls outside of an expected range of impedancevalues.
 7. The method of claim 1, wherein the electrical signalcomprises an electrical signal with a frequency of approximately onekilohertz to approximately 50 kilohertz.
 8. The method of claim 1,wherein monitoring the electrical signal in the component of the machinecomprises monitoring the component for an electrical signal having afrequency that is within a predetermined frequency range.
 9. A systemfor monitoring a machine, the system comprising: a signal generatorconfigured to provide an electrical signal that is transmitted to andinto at least one component of the machine; one or more detectiondevices configured to detect a capacitance associated with theelectrical signal in the at least one component; and at least oneprocessor configured to: compare the monitored capacitance to at leastone expected value; and detect, based at least in part on thecomparison, the occurrence of a potential rub condition for thecomponent.
 10. The system of claim 9, wherein the machine comprises aturbine.
 11. The system of claim 9, wherein the electrical signalcomprises an alternating current voltage signal comprising a voltage ofapproximately one to approximately two volts.
 12. The system of claim 9,wherein the at least one processor is further configured to: determinethat the monitored capacitance exceeds a threshold value; and detect theoccurrence of the potential rub condition based at least in part on thedetermination.
 13. The system of claim 9, wherein the at least oneprocessor is further configured to: monitor an impedance associated withthe electrical signal; and detect the occurrence of a rub conditionbased at least in part on detecting a decrease in the monitoredimpedance.
 14. The system of claim 9, wherein the at least one processoris further configured to determine a severity associated with thedetected rub condition based at least in part on a length of time forwhich the monitored impedance falls outside of an expected range ofimpedance values.
 15. The system of claim 9, wherein the electricalsignal comprises an electrical signal with a frequency of approximatelyone kilohertz to approximately 50 kilohertz.
 16. The system of claim 9,wherein the one or more detection devices are operable to detectelectrical signals that fall within a predetermined frequency range. 17.A method for monitoring a machine, the method comprising: transmittingan alternating current voltage signal to and into at least one componentof the machine; monitoring a capacitance associated with the signal;comparing the monitored capacitance to at least one expected value; anddetecting, based at least in part on the comparison, the occurrence of apotential rub condition for the component.
 18. The method of claim 17,wherein the alternating current voltage signal comprises a voltage ofapproximately one to approximately two volts.
 19. The method of claim17, further comprising: monitoring an impedance associated with thesignal, and detecting a rub condition based at least in part ondetermining a decrease in the monitored impedance.
 20. The method ofclaim 17, further comprising determining a severity associated with theidentified rub condition based at least in part on a length of time forwhich the determined impedance falls outside of an expected range ofimpedance values.